1. Field of the Invention
The present invention relates to non-invasive measurement of internal information in a scattering medium by causing pulsed light, square wave light, or continuous light to be incident on a scattering medium such as a living body, and detecting light propagating in the scattering medium. More particularly present invention relates to a method for measuring internal information in a scattering medium and an apparatus for the same, which are capable of measuring an absolute value of an absorption coefficient and a transport scattering coefficient of the scattering medium, the concentration of a specific absorptive constituent in the scattering medium, its time change, and its spatial distribution.
2. Related Background Art
Demands for precise measurement of an absorption coefficient or a transport scattering coefficient in a scattering medium such as a living body or the precise measurement of the concentration of a specific absorptive constituent are very strong, and there have been several reports and attempts for such measurement. The main prior arts are listed as references 1.-5. at the end of this section.
In general, since light is scattered and/or absorbed at random in a scattering medium, light does not propagate in a straight line. For a scattering medium in which the absorption is zero, the total quantity of light never decreases, but since light is scattered by scattering constituents at random, light propagates in a zigzag manner. In this case, the distance that light can propagate without any effect of scattering is called a mean free path or a mean diffusion length, which is the inverse of a transport scattering coefficient .mu..sub.s '. In the case of the living body specimen, the mean free path is about 2 mm. Such is disclosed in Wilson et al., Optical Reflectance and Transmittance of Tissues, Principle and Application, IEEE J. Quantum Electron, Vol. 26, No. 12, pp. 2186-2199. In addition, the scattering medium comprises absorptive constituents other than the scattering constituents, so that the absorption occurs in accordance with a distance that light propagates while scattering, and the quantity of light is exponentially attenuated in accordance with the distance.
As the prior art in the field of precise measurement of an absorption coefficient or a transport scattering coefficient of a scattering medium, one document teaches measuring the quantity of transmitted light or reflected light corresponding to continuous light or pulsed light incidence, and one document teaches measuring the transmitted light or reflected light corresponding to pulsed light incidence and to analyze its waveform. The former utilizes a measurement of the absorbance in which the Lambert-Beer Law is a basic principle, and there a further document utilizes a principle of dual-wavelength spectroscopy. Here, Lambert-Beer law is that the absorbance of the specimen is proportional to the product of the molar absorption coefficient, molar concentration, and a specimen thickness, and that a difference of the absorbances is proportional to the concentration difference where a thickness of the specimen is constant.
However, in the case of the absorbance optical density measurement in the scattering medium, a mean optical pathlength of light diffused during propagation between a light incident position and a photodetection point is varied depending on the absorption coefficient .mu..sub.a of the scattering medium. Therefore, in the absorbance measurement corresponding to the scattering medium in which the optical pathlength is constant, an absorption coefficient dependency of the mean optical pathlength is a big problem, which hinders the precise measurement of an absorption coefficient or the concentration of an absorptive constituent, or if the measurement is performed, due to large errors in measurement, it cannot practically be used. For example, according to "Japanese Patent Application No. Sho 62-248590", "Japanese Patent Application No. Sho 62-336197", and "Japanese Patent Application No. Hei 2-231378", since a basic principle is to measure an optical coefficient in which an optical pathlength is assumed to be constant, errors in measurement caused by a change of the above-described optical pathlength cannot be avoided.
There is another method utilizing a mean optical pathlength which is measured by another method in the case of an absorbance measurement, bun hence a mean optical pathlength is varied depending on the absorption coefficient, errors caused by approximating the average optical pathlength to be a constant value cannot be avoided. Further, there are a method of measuring an absorbance difference using pulsed light, a method further utilizing a principle of dual-wavelength spectroscopy, and a method of measuring an absorbance optical density using light having three or more kinds of wavelengths, but in either case, since a method of measuring an absorbance in which an optical pathlength in a scattering medium is assumed to be constant is applied, errors caused by the change of the optical pathlength occur in the all cases, so that the sufficient precise measurement can not be performed.
In the prior art not based on an absorbance measurement, a method, in which transmitted light or reflected light is measured by time-resolved measurement using pulsed light or modulated light to analyze waveform, has disadvantages that owing to the time-resolved measurement, the measurement method and the apparatus are very complicated and the apparatus is expensive, as compared with a measurement method and an apparatus according to the present invention in which quantity of light, that is, time integration of an optical signal is measured. There are several attempts for measuring internal absorption information by measuring reflected light or transmitted light upon the incidence of pulsed light to the medium by the time-resolved measurement and analyzing its waveform. This is suggested in Patterson et al., "Time Resolved Reflectance and Transmittance, for the Non-Invasive Measurement of Tissue Optical Properties, Applied Optics, Vol. 28, No. 12, pp. 2331-2336 (1989); Patterson et al., Applications of Time-Resolved Light Scattering Measurements to Photodynamic Therapy Dosimetry, Proc. SPIE, Vol.1203, pp. 62-75 (1990); and Sevick et al., Time-Dependent Photon Migration Imaging, Proc. SPIE, Vol. 1599, pp. 273-283 (1991). A measured optical signal has a long decay tail by the influence of scattering and absorption constituents. Patterson et al. assumes a model of uniform scattering medium to analytically obtain the light signal output. See Patterson et al. (1989). A wave representing a time change in intensity of the optical output signal given by the formula defined by Patterson et al. matches a waveform obtained by an experiment using a uniform scattering medium. According to Patterson et al. and the results of the experiment by the inventor of the present application, the absorption coefficient of absorptive constituents in the scattering medium is given by a slope of a waveform (differential coefficient) obtained when the optical signal is sufficiently attenuated, i.e., when a sufficiently long period has elapsed. However, because the optical signal at a location where an absorption coefficient is obtained required to be sufficiently attenuated means that the signal is very feeble, a signal to noise ratio (S/N) of the signal to measured is decreased, and consequently errors in measurement are increased. Therefore, such a system and method are hard to use in practical application. There are several kinds of attempts other than the above, but none of them gives sufficient measurement precision.
A method which utilizes the above-stated dual-wavelength spectroscopy measurement to a scattering medium has the following problems. In the absorbance measurement of a scattering medium, an extinction coefficient is a sum of a transport scattering coefficient and an absorption coefficient by definition. Since these parameters are treated at the same level, the absorption influence, e.g., an absorption coefficient, cannot be measured while scattering and absorption influences cannot be separated. In general, this can be solved by using a principle of dual-wavelength spectroscopy measurement, an absorption coefficient is measured by using two or more suitable kinds of lights having different absorption coefficients resulting in different absorption constituents, and assuming that a scattering coefficient and a transport scattering coefficient of the two or more kinds of lights are the same, or if they are different, assuming that the difference is very small, the scattering influence is eliminated from an absorbance optical density difference corresponding to two or more kinds of light to obtain the concentration of an absorption constituent or an absorption coefficient. This method has a disadvantage of the occurrence of errors caused by the assumption that scattering coefficients or transport scattering coefficients corresponding to different wavelengths are equal.